Many environmental factors may affect the shape and other characteristics
of bacteria (e.g. pigmentation, growth rate, size)
a. temperature of incubation
b. age of the culture (not an individual organism)
c. composition and concentration of the substrate
d. method of fixing and staining
e. pH and oxygen concentration
f. toxic waste buildup
Basic shape is determined by heredity
1. rod bacillus
2. round coccus
3. spiral spirillum
Shapes are part of a continuum rather than being absolute;
some bacteria are naturally pleomorphic (multiple shapes)
Bacteria usually exhibit characteristic morphology in young
cultures (24 hours) and on favorable growth media. Young cells are often larger than older cells of the same species
CAPSULES -
organized accumulations of gelatinous materials exterior to the
cell wall; in contrast to SLIME LAYERS (also known as the GLYCOCALYX)
which are unorganized accumulations of similar materials
production is genetically controlled and subject to environmental
modification wide range in density, thickness, and adhesiveness
among different strains; probably produced by the cell membrane
capsules have varying chemical composition dependent on species:
may consist of glucose polymers, complex polysaccharides, amino
sugars, sugar acids, polypeptides alone or in combination
Functions:
1. since capsular material is about 98 % water, capsules probably
serve as defense buffers against too rapid influx of water
into the cell and also against dehydration
i.e. osmotic buffers
2. sometimes associated with pathogenicity
3. confer immunological specificity
4. protects bacteria from ingestion by phagocytic cells (greased
pig analogy), and bactericidal factors in body fluids of host
SOME EXAMPLES of encapsulated pathogens:
Bacillus anthracis - anthrax
Clostridium perfringens - gas gangrene, food poisoning
Streptococcus pneumoniae - pneumonia
Klebsiella pneumoniae - pneumonia
CELL WALL -
essential to bacterial cell; rigid structure does not allow for expansion
chemically unlike any structure present in animal tissues thus an obvious target for drugs that can attack and kill bacteria without harm to the host
provides strong, rigid structural component that can withstand
high osmotic pressure caused by high chemical concentrations of
inorganic ions in cell (may be as high as 25 atmospheres of
pressure);
Gram cells can usually withstand greater internal
atmospheric pressure than Gram + cells
all bacterial cells have common structural component commonly
called PEPTIDOGLYCAN, murein, cell wall mucopolysaccharide which consists of amino acids tied into a structure which is not a protein:
Peptidoglycan is a large, sheetlike molecule (like a piece of burlap)which
covers the entire cell
Cell Wall Differences: Gram + Thick, 2080 nanometers, 60100 % peptidoglycan; all are linear polymers of NAG & NAM but vary in length and composition of peptides used to bridge NAG & NAM
some contain TEICHOIC ACIDS linked to NAM;
highly antigenic, aid
in serological identification;
ALL contain GLYCEROL (3 carbon
sugar alcohol) which combines with lipid membrane structure and
is involved in regulation of normal cell division;
protein is NOT found as a constituent of Gram + cell walls
except for Group A streptococci which contain M protein
Gram - Thinner, more complex; less peptidoglycan (1020 %);
exterior to the peptidoglycan is a membrane composed of protein and
lipopolysaccharide (fatty acids linked to polysaccharide); together
this membrane and the peptidoglycan make up the cell wall; this complex
is important because it may cause toxicity in animals, particularly
inflammation known as ENDOTOXIN lipopolysaccharide retains its
toxicity when dissociated from the protein and may serve in the regulation
of ions passing into the cell.
CELL MEMBRANE
fragile, located interior to cell wall; accounts for 810 % of dry weight of the cell
chemically composed of phospholipids (glycerol, fatty acids,
and phosphate) with proteins embedded in it;
structurally similar
to eukaryotic cell membrane and carries on similar passive and
active transport activities
about 40% lipid, 60% protein, less than 10% carbohydrates
invagination of the membrane provides the cell's only means of
expanding its surface area
Functions:
1. in aerobic organisms, it transports electrons and protons
released during oxidation of bacterial foodstuffs to oxygen
with subsequent formation of water; it also converts energy
from such oxidations into chemical energy
2. contains some of the enzymes necessary for synthesis and
transport of peptidoglycan, teichoic acid, and outer membrane
components
3. secretes extracellular hydrolytic enzymes
4. ensures segregation of DNA to daughter cells during cell
division
5. controls transport of most compounds entering & leaving cell
(diffusion, osmosis, facilitated & active transport utilizes
permease enzymes)
PILI or FIMBRIAE
fine, filamentous surface appendages of varying diameters and
lengths extending outward from the surface of bacteria
can be seen only with electron microscope;
occur most frequently
in Gram rods;
originate inside of cell just below the cell membrane;
production is genetically controlled
consist of a unique protein known as PILIN, arranged
helically to form a single rigid filament with a hollow core
Functions:
1. allows for adherence to most surfaces, cellular or otherwise;
may cause hemagglutination of human and animal red blood cells
2. attachment of two bacteria prior to transfer of DNA from one
to the other requires special longer F pilus
3. allows organisms to form surface film (pellicle) which could
enhance microbial growth in nonmoving culture situations
where oxygen supply is limited
4. used as receptor sites for bacterial viruses (bacteriophages)
MESOSOMES - (probably artifacts of staining procedure)
the following is information often found in texts
intracellular membranous structures, first seen in 1957; they
appear as pocketlike structures that contain tubules, vesicles or lamellae;
chemically identical to the cell membrane
a folded membrane arrangement is found most often in Gram +
Functions:
1. increase cell's membrane surface, which in turn increases
enzymatic needs of cell and formation of new mesosomes
2. mesosome formation may precede and coordinate cross wall
formation prior to cell division and spore formation (cell
wall and endospore formation)
3. mesosomes may adhere to nuclear material (DNA) and act as a
primitive mitotic apparatus to insure that each daughter cell
receives appropriate share of DNA; does not appear to be
essential for the process to occur
4. increases surface area for respiratory activity
5. may take the place of the ENDOPLASMIC RETICULUM and GOLGI
FLAGELLA (singular FLAGELLUM)
provides for true motility as opposed to Brownian movement;
threadlike appendage; usually several times the length of the
cell; small size cannot be seen in routine stained smears;
special staining methods coat the flagella or cause it to swell
originate just below the cell membrane; attached by a hook and a
series of plates or rings which anchor the flagellum to the cell
wall and membrane 24 rings, only 2 in Gram + cells because of
thicker cell wall (only S and M rings)
flagella consist of three parallel protein fibers intertwined
into a triple helical structure; protein is called FLAGELLIN
(contains an amino acid found nowhere else EpsilonNmethyl
lysine); Flagellin is actually a family of proteins rather than a single one
the amino acid composition of Flagellin varies for each species
and thus confers immunological specificity for each species
flagella can be broken off and will regenerate; may be
dissociated and will repolymerize capable of auto (self)
assembly; energy source for flagella is presently unidentified
Functions:
1. allows organisms to migrate toward favorable growth
environments and away from those that might be harmful
2. may increase the concentration of nutrients or decrease the
concentration of poisonous materials near bacterial surfaces
by causing change of flow rate of environmental fluids
3. can disperse flagellated organisms to uninhabited areas
where colony formation can be achieved
4. confer immunological specificity on species
5. may enable flagellated pathogens to more easily penetrate
host defenses, such as mucous secretions
the most satisfactory motility is observed in young cultures
(1824 hours or less); bacteria tend to become nonmotile in
older cultures because of crowding with inert living and dead
bacteria; production of acid and other toxic products may cause
loss of motility in older cultures
Flagella located at one or both ends of the cell only are called
POLAR; if the entire cell surface is covered with flagella it is
said to be a PERITRICHOUS condition
CYTOSKELETON
prior to the early 1990's it was assumed that bacteria did NOT possess a cytoskeletal network of internal support; in the 1990's evidence emerged that bacteria possessed their own version (homolog) of actin protein; tubulin and intermediate filament homologs have since been found (Naturwissenshaften 1998 Jun;85(6):278-82 and Cell Biology Int. 2003;27(7):503-5 - additional sources may be found through a Google search); current evidence shows that there IS a dynamic cytoskeleton present within the bacterial cytosol
NUCLEAR REGION (NUCLEOID)
not membrane limited; lacks associated histones; exhibits no
mitotic or meiotic phenomena or apparatus; no nucleolus
DNA is contained within a single, long, doublestranded, circular
chromosome (closed loop); may have as many as 6 million
nucleotide bases (Escherichia coli) - able to produce over 20,000 enzymes;
coded information is overlapped within the chromosome (Scientific American,
January 1966).
Additional genetic information is found in EXTRACHROMOSOMAL circular DNA
molecules known as PLASMIDS; they are capable of independent replication
and carry information for a variety of functions, e.g. drug resistance,
production of bacteriocins
metabolic PLASMIDS of Pseudomonas produce metabolic enzymes
some PLASMIDS are capable of shifting their mode of replication; they can
move back and forth between the bacterial chromosome and their normal site
near the cell membrane; when they enter the chromosome they become part of
it and are replicated when the bacterium carries on binary fission; as
PLASMIDS they can reproduce on demand within the non-dividing cell
these PLASMIDS are given different names (to confuse students); they are
sometimes called BIPHASIC REPLICONS of EXTRACHROMOSOMAL DNA - the most
common term applied to them is EPISOMES; occasionally both PLASMIDS and
EPISOMES are referred to as TRANSPOSONS
RIBOSOMES
scattered throughout the cytoplasm; consist of about 40% protein and 60% ribosomal RNA;
smaller than those in eukaryotic cells but similar in size to those in the mitochondria
and chloroplasts
there is no conspicuous ENDOPLASMIC RETICULUM within the bacterial cell
ribosomes sometimes are found in groups or aggregates known as POLYRIBOSOMES or POLYSOMES
ENDOSPORES
dormant structure formed inside of individual bacterium; resistant to most
adverse environmental factors such as: temperature (heat and cold), dessication
(dehydration). chemicals, radiation, common dyes
Chemical Composition
little free or unbound water; peptidoglycan, DIPICOLINIC ACID
and calcium ions - the last two are usually bound together as
calcium dipicolinate (reduced form of the acid with the
calcium replacing the hydrogen)
DPA = 2,6 pyridine dicarboxylic acid
DPA is most responsible for heat resistance
core contains characteristic quantity of DNA, small amounts of
enzymes and RNA; no messenger RNA; high concentrations of
cystine (amino acid) in outer coat - permits cross linkage and
has been shown to be effective in resistance to radiation
Sporulation proceeds when conditions are no longer favorable for growth.
The following chemical substances appear to be required for sporulation:
1. glucose - energy source
2. particular amino acids - needed for structural molecules
3. growth factors - vitamins and minerals, including folic
acid, phosphate, calcium, manganese, bicarbonate (buffer
to maintain pH balance)
Endospore is NOT a reproductive cell; more like fall-out shelter;
serves as survival mechanism
Medically important spore formers:
the genus Bacillus and the genus Clostridium
other spore formers include:
Sporosarcina - only coccal spore former known; marine organisms
Sporolactobacillus - microaerophilic, found in chicken feed
Desulfomaculum - anaerobe, found in soil, water, some intestines
of insects and cows
CYTOPLASMIC INCLUSIONS (Inert)
bacteria accumulate reserve materials - both water soluble and water insoluble;
they are non-living and most frequently include lipids, polysaccharides, and
certain inorganic materials; these inclusions are divided into two major categories
I. Non-membrane enclosed inclusions
A. Metachromatic granules - sometimes called Babes-Ernst
granules or VOLUTIN; they possess an affinity for basic
dyes and thus cause color changes within the cell, e.g.
these areas turn red when methylene blue is added; they
usually contain various associations of phosphates (inor-
ganic), nucleic acid, lipids, and proteins; they probably
serve as temporary storage of reserve materials
B. Polysaccharide granules - usually insoluble polysaccharides;
most commonly glucose polymers
II. Membrane-enclosed Inclusions - surrounded by a single layer
of protein
A. Carboxysomes - restricted to photosynthetic cells;
contain enzymes responsible for carbon dioxide fixation
B. Lipid Inclusions - appear most regularly in Gram +
species, more prominent as the cell ages; usually a
polymerized form of the fatty acid B-hydroxybutyric acid
(BHA); found in butter, used a food preservative; may
become poly b-hydroxybutyric acid and hydrolyzed back to
soluble BHA
C. Sulfur Globules - pure elemental sulfur may accumulate as
unused food; derived from intracellular dehydrogenation
(oxidation) of hydrogen sulfide or other inorganic chemi-
cally reduced forms of sulfur
D. Gas Vacuoles - usually in aquatic procaryotes; provides
for provision and regulation of cell buoyancy, light-
shielding, and surface to volume regulation
Advantages of Storing Food as Polymers
1. many molecules may be condensed into a small area
2. can be made available for cell use by simple hydrolysis
into soluble units
3. polymers do not affect the intracellular osmotic pressure
or other properties which are normally affected by the
number of particles in suspension or solution
